In recent years, the sizes of liquid crystal displays continue to increase, and the frequency of a display drive circuit is increased as well. The mobility of amorphous silicon thin film transistors (TFTs) is about 0.5 cmW·S. When the size of a liquid crystal display is over 80 inches and the drive frequency is 120 Hz, the mobility of the TFTs used in the liquid crystal display is required to be above 1 cmW·S. Therefore, the mobility of amorphous silicon TFTs fails to meet the requirement obviously. Thin film transistors with high mobility comprise polysilicon thin film transistors and metal oxide thin film transistors. However, polysilicon thin film transistors suffer poor uniformity in manufacturing, and the manufacturing process is sophisticated. Indium gallium zinc oxide thin film transistors (IGZO-TFTs) possess high mobility, good uniformity, transparency, and simple manufacturing process, and can meet the demand for a large-sized liquid crystal display and for active organic electroluminescent.
The structures of current metal oxide IGZO-TFTs mainly involve three types: an etching barrier type, a back channel etching type, and a coplanar type.
The metal oxide IGZO-TFT of the etching barrier type has a simple manufacturing process. The etching barrier layer on the metal oxide IGZO layer can protect the metal oxide IGZO layer from being damaged when source/drain metal electrodes are under formation. Therefore, the performance of the metal oxide IGZO-TFT is improved. However, this process requires an additional photolithography process to form the etching barrier layer, which is an extra process for manufacturing a metal oxide IGZO-TFT.
Since the manufacturing process of an etching barrier type TFT requires to etching a barrier layer and correspondingly requires a mask process for this layer, it complicates the whole manufacturing process, increases costs for products, decreases producing capacity and yield, and eventually lowers the profit of the product.